feat(if-else): add if-else access control

listings/ch00-introduction/if_else/.gitignore

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+target

listings/ch00-introduction/if_else/Scarb.toml

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+[package]
+name = "if_else"
+version = "0.1.0"
+
+[dependencies]
+starknet = "1.1.0"
+
+[[target.starknet-contract]]

listings/ch00-introduction/if_else/src/access_log.cairo

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+#[contract]
+mod SimpleAccessLog {
+    // Import the Starknet contract API.
+    use starknet::{get_caller_address, ContractAddress};
+
+    struct Storage {
+        // Add the owner to the contract storage.
+        _owner: ContractAddress,
+    }
+
+    #[event]
+    // Add a welcome event to the contract.
+    fn WelcomeEvent(name: felt252) {}
+
+    #[constructor]
+    fn constructor(address: ContractAddress) {
+        // Set the owner to be the address that deployed the contract.
+        _owner::write(address);
+    }
+
+    // Add a function that checks if the caller is the owner.
+    fn is_owner() -> bool {
+        return get_caller_address() == _owner::read();
+    }
+
+    #[external]
+    fn log_access() {
+        //  Add a conditional event that welcomes the owner or the user.
+        if (is_owner()) {
+            // We know since is_owner() == true, the owner called the function. Call the welcome event with 'Welcome Admin!'.
+            WelcomeEvent('Welcome Admin!');
+        } else {
+            // We know since is_owner() == false, a normal user(not owner) called the function. Call the welcome event with 'Welcome User!'.
+            WelcomeEvent('Welcome User!');
+        }
+    }
+}

listings/ch00-introduction/if_else/src/lib.cairo

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+mod access_log;

src/SUMMARY.md

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   - [Mappings](./ch00-06-mappings.md)
   - [Constructors](./ch00-07-constructor.md)
   - [Function Attributes](./ch00-08-function_attributes.md)
+  - [If statements](./ch00-09-if_statements.md)
 
 - [Applications](./ch01-00-applications.md)
   - [Upgradeable Contract](./ch01-01-upgradeable_contract.md)

src/ch00-09-if_statements.md

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+# If Statements
+
+Using an if expression allows you to execute certain code depending on conditions. Within the if statement provide a condition and then specify what you want to execute within the block of code. You can specify an else expression which is reached if the If condition is not met.
+
+Here's a simple example of a contract using if and else statements for role based access log:
+
+The contract will log a welcome message to the console depending on who called the function. If the owner called the function, the welcome message will be 'Welcome Admin!'. If a normal user called the function, the welcome message will be 'Welcome User!'.
+
+```rust
+{{#include ../listings/ch00-introduction/if_else/src/access_log.cairo}}
+```